There was a potentially useful thread a while back: https://forum.atlas-games.com/t/shape-of-the-fire-lord-aka-muto-dragon/5950/1 , although it doesn't really cover virtues and mysteries.
A loose summary is that turning yourself into a dragon shaped creature isn't too tricky (at least, to design - it's probably getting highish in level, especially once you start adding magnitudes for size). I reckon MuCo(An) should do it, before you start thinking about firebreath.
After that, it starts depending on what you mean by "a dragon". Getting firebreath is doable in several ways - the most classically "dragon" is to add a CrIg requisite to the original spell, but this is likely to lead to a yet higher original spell level and poor penetration. Alternatives include spells and enchanted items cast/triggered separately by the "dragon", possibly with the cosmetic effect of coming out from the dragon's mouth (I reckon you can do this without burns in the same way that Ball of Abysmal Flame seems to fly from your hand without singing it).
Getting might is a lot harder, if you want it. Standard Hermetic magic can't grant it. I'm not sure there's any fundemental reason you can't combine any of several of the ways of becoming an immortal being with a might score that are in the Mysteries: Revised Edition with a separate spell to become dragon shaped, though, although they usually tend to be fairly irrevocable, to have substantial downsides and to be the culmination of a lifetime's achievement.
Beyond that, Ignem Monstrosity from Realms of Power: Magic can give you firey breath, although as it's a flaw, you'll probably have limited ability to turn it off.